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Plant Physiol, June 2000, Vol. 123, pp. 671-680
Variation in the Oxygen Isotope Ratio of Phloem Sap Sucrose from
Castor Bean. Evidence in Support of the Péclet
Effect1
Margaret M.
Barbour,
Ulrich
Schurr,
Beverley K.
Henry,2
S. Chin
Wong, and
Graham D.
Farquhar*
Environmental Biology Group, Research School of Biological
Sciences, Institute of Advanced Studies, Australian National
University, G.P.O. Box 475, Australian Capital Territory 2601, Australia (M.M.B., U.S., B.K.H., S.C.W., G.D.F.); and Botanical
Institute, University Heidelberg, Im Neuenheimer Feld 360, 69120 Heidelberg, Germany (U.S.)
Theory suggests that the level of enrichment of
18O above source water in plant organic material ( ) may
provide an integrative indicator of control of water loss. However,
there are still gaps in our understanding of the processes affecting
. One such gap is the observed discrepancy between modeled
enrichment of water at the sites of evaporation within the leaf and
measured enrichment of the leaf water as a whole (L).
Farquhar and Lloyd (1993) suggested that this may be caused by a
Péclet effect. It is also unclear whether organic material formed
in the leaf reflects enrichment of water at the sites of evaporation
within the leaf or L. To investigate this question
castor bean (Ricinus communis L.) leaves, still attached
to the plant, were sealed into a controlled-environment gas exchange
chamber and subjected to a step change in leaf-to-air vapor pressure
difference. Sucrose was collected from a cut on the petiole of the leaf
in the chamber under equilibrium conditions and every hour for 6 h
after the change in leaf-to-air vapor pressure difference. Oxygen
isotope composition of sucrose in the phloem sap (suc)
reflected modeled L. A model is presented describing suc at isotopic steady state, and accounts for 96% of
variation in measured suc. The data strongly support the
Péclet effect theory.
1
This work was supported by Micromass UK Ltd. and
by the Australian National University with a visiting fellowship to the
Research School of Biological Sciences (to U.S.).
2
Present address: Queensland Department of
Natural Resources, 80 Meiers Road, Indooroopilly, QLD 4068, Australia.
*
Corresponding author; e-mail farquhar{at}rsbs.anu.edu.au; fax
61-2-6249-4919.
© 2000 American Society of Plant Physiologists
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